fix: limit test job to single architecture build

- Use :core:testDebugUnitTest for targeted test execution
- Add CARGO_NDK_TARGET_ARCH=arm64 to prevent building all 4 archs
- Reduces disk usage by ~75% (1 arch vs 4 archs)

Tests still depend on Android NDK builds for UniFFI bindings
generation, so we need the environment variable to limit targets.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: b-zee

.github/workflows/android.yml

@@ -89,8 +89,10 @@ jobs:
       working-directory: android
 
     - name: Unit test
-      run: ./gradlew test
+      run: ./gradlew :core:testDebugUnitTest
       working-directory: android
+      env:
+        CARGO_NDK_TARGET_ARCH: arm64
 
     - name: 'Upload Artifact'
       uses: actions/upload-artifact@v4